Direct observation of the steric effect in chemical reactions by controlling the orientation at transition state

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Direct observation of the steric effect in the OH + HBr chemical reaction was made by controlling the OH radical orientation in order to clarify mechanisms for stereo-anisotropy. We find that the HBr collision at the O-end of OH radical is more favorable than that at the H-end. The theoretical analysis suggested that there are at least two mechanisms play roles to give the observed steric effect. One is the H atom abstraction at the O-end of OH and another is the H atom migration at the H-end of OH. Alignment of CO(a) was performed using a linearly polarized laser at 206nm. This success will allow us to control molecular orientation at transition state in a novel way.